This invention relates to an optically switchable device comprising a heterostructure defining a first potential well separated by a barrier layer from a second potential well which provides an electron energy level which is lower in electron energy than the lowest electron energy level of the first potential well.
Such an optically switchable device is described in an article by B. Deveaud et al entitled `An ultra-fast Optical Modulator: the Double-Well GaAs/GaAlAs Superlattice (DWSL)` published at pages 101 to 103 of the proceedings of the High Speed Electronics International Conference 1986.
As described in the aforementioned article, the second potential well is wider than the first potential well so as to provide an electron energy level lower in electron energy than the lowest electron energy level of the first potential well and the first and second potential wells are closely associated through a thin barrier layer. The relatively narrow first potential well is used as the optically active part of the device where the excitonic absorption can be bleached or saturated very rapidly by an external light source or optical beam. This enables the device to be used as an optical switch which normally absorbs light at the wavelength of the exciton resonance but which allows transmission of such light when the exciton absorption is bleached or saturated, that is when the energy levels of the first potential well are filled by the photo-generated carriers.
In the device described in the aforementioned paper, the relatively wide second potential well is very closely associated with the first potential well and rapidly traps excess carriers in a time typically less than 10 ps (picoseconds) allowing the optical device to return rapidly to the normal steady state in which the exciton resonance is restored when the optical beam which caused the exciton bleaching is switched off or its power density level reduced below that sufficient to sustain bleaching. However, because of the fact that the excess carriers rapidly recombine, an optical beam with a very considerable power density level is required in order to achieve the desired exciton bleaching. Where large parallel arrays of such optical devices are desired, for example for optical computing or image or optical data processing, then the dissipation of the large amount of heat generated by such high power density levels can present problems.